Thanks to everyones ideas & input and
@Bohdan for the info.
To the metric guys out there and especially Australians 12mm x 3mm stainless steel is available at most stainless steel merchant
I bought 4m bar from merchant

as that was there minimum supply plus $4 a cut!
Would have been cheaper from these guys inc $13 delivery
http://allthingsstainless.com.au/virtuemart/stainless-steel-flat-bar/304-12mm-x-3mm-stainless-steel-flat-bar-per-metre.html500mm x 4 bars (all 3mm thick). They fit nice and snug. 8 x M5 6mm set/grub screws from any nuts & bolts shop $2 for 8 or if you're desperate - Bunnings for $6.48 for 10
I bought a Tap & Die set last week from Aldi for $29.00 as I knew I needed one for this project. It worked a treat as I'd never done this stuff before. I didnt bother with the 2 screws to hold it together as the bars are clamped nice and tight.
One note that they do sell Mild steel but was 13mm x 3mm - it might fit and would be easier to cut and a lot cheaper.
HTH anyone who is thinking of making them - well worth it and the bonus as we all know - save money and the possibility of damaged rails
Cheers